Wash. Rev. Code § 63.14.156
The holder of a retail installment contract may, upon agreement with the buyer, extend the scheduled due date or defer a scheduled payment of all or of any part of any installment or installments payable thereunder. No charge shall be made for any such extension or deferment unless a written acknowledgment of such extension or deferment is sent or delivered to the buyer. The holder may charge and contract for the payment of an extension or deferral charge by the buyer and collect and receive the same, but such charge may not exceed those permitted by *RCW 63.14.130 (a), (b), or (c) on the amount of the installment or installments, or part thereof, extended or deferred for the period of extension or deferral. Such period shall not exceed the period from the date when such extended or deferred installment or installments, or part thereof, would have been payable in the absence of such extension or deferral, to the date when such installment or installments, or part thereof, are made payable under the agreement of extension or deferment; except that a minimum charge of one dollar for the period of extension or deferral may be made in any case where the extension or deferral charge, when computed at such rate, amounts to less than one dollar. Such agreement may also provide for the payment by the buyer of the additional cost to the holder of the contract of premiums for continuing in force, until the end of such period of extension or deferral, any insurance coverages provided for in the contract, subject to the provisions of RCW 63.14.140.
RCW 63.14.156
*Reviser's note: The reference to RCW 63.14.130 (a), (b), or (c) is erroneous. RCW 63.14.130(1) (a) or (b) is apparently intended. Subsequently, RCW 63.14.130 was amended by 1992 c 193 s 2, changing the subsection numbering.